1. Field of the Invention
The present invention relates to a spectrometer and an optical spectrum analyzer using a chromatic dispersion device, and more specifically, to a spectrometer and an optical spectrum analyzer whereby the accuracy of wavelength measurement can be improved without being affected by the environment of use.
2. Description of the Prior Art
A spectrometer spectrally divides the light under measurement by transmitting the components thereof at different, wavelength-by-wavelength angles using a chromatic dispersion device, and detects the light thus spectrally divided by the chromatic dispersion device, using an optical detector. An optical spectrum analyzer uses an output from the optical detector of the spectrometer to measure the wavelengths of optical signals (for example, Japanese Laid-open Patent Application 2000-304613). For example, the analyzer is used for such light under measurement in which a plurality of optical signals are wavelength-division multiplexed (WDM), in order to measure the wavelengths of the individual optical signals.
FIG. 1 is a schematic view illustrating an example of such a conventional optical spectrum analyzer as mentioned above.
In FIG. 1, spectrometer 10 is given an input of light under measurement, measures the spectrum of the light, and outputs measurement data as sampling data. Spectrometer 10 comprises optical fiber 11, collimating lens 12, diffraction grating 13, focusing lens 14, and photodiode array module 15.
Optical fiber 11 is a transmission line having an optical output window for emitting the light to be measured. Collimating lens 12 is positioned opposite to the optical output window of optical fiber 11, in order to collimate the light to be measured emitted from optical fiber 11 before transmitting the light.
Diffraction grating 13 is a chromatic dispersion device, which is tilted toward collimating lens 12 so that outgoing light from collimating lens 12 is diffracted at a desired angle. Diffraction grating 13 spectrally divides the light under measurement by reflecting the components thereof at different, wavelength-by-wavelength angles. Focusing lens 14, which is placed on the optical path of the outgoing light from diffraction grating 13, converges the outgoing light to form an image.
Photodiode array module 15 (hereinafter abbreviated as PDM 15) is an optical detector comprising a plurality of photodiodes that are light receiving elements, and is placed in a position where the light under measurement is converged to form an image.
PDM 15 samples the optical power of the light under measurement using the light receiving elements and outputs sampling data as the measurement data. Wavelengths are previously allocated to the individual light receiving elements of PDM 15.
Memory 20 is a storage unit and stores measurement data output from spectrometer 10. Wavelength calculation means 30 reads the measurement data from memory 20 and calculates the wavelengths of optical signals from the wavelengths allocated to the individual light receiving elements of PDM 15.
The behavior of the apparatus configured as described above is explained below.
The light under measurement emitted from optical fiber 11 is collimated by collimating lens 12. The light under measurement that has passed through collimating lens 12 enters diffraction grating 13. The light under measurement is then spectrally divided by diffraction grating 13. In other words, the angle of reflection from diffraction grating 13 differs depending on the wavelength of each ray of the light. The components of the light under measurement that has been spectrally divided by diffraction grating 13 are converged by focusing lens 14 at the individual light receiving elements of PDM 15 to form images.
For example, the components of light with different wavelengths are converged by focusing lens 14 on the light receiving elements positioned at points “FP01,” “FP02” and “FP03” in FIG. 1 to form images.
Each light receiving element of PDM 15 outputs a current (photocurrent) corresponding to the optical power of each ray of the light under measurement. Using a converter which is not illustrated in the figure, PDM 15 converts photocurrents output from the individual light receiving elements to voltages. Since the signals obtained by current-to-voltage conversion are analog signals, the converter converts the analog signals to digital signals, and the digital signals are stored in memory 20 as measurement data.
As explained above, the measurement data composes sampling data that has been sampled by using the light receiving elements.
Wavelength calculation means 30 reads the measurement data from memory 20, determines the wavelengths of optical signals from the wavelengths allocated to the individual light receiving elements, and outputs these results of calculation to an output unit not illustrated in the figure. The output unit displays the calculation results on a display, for example, or outputs the results to an external device not illustrated in the figure.
Next, the relationship between the incidence and reflection angles of the light under measurement formed by diffraction grating 13 is explained below.
The relationship between the incidence and reflection angles of the light under measurement formed by diffraction grating 13 is represented by equation (1) below.sin θgi+sin θgo=λ/(na·d)  (1)where θgi is the angle of incidence of the light under measurement toward diffraction grating 13, θgo is the angle of reflection of the light under measurement from diffraction grating 13, λ is the wavelength, na is the refractive index of the medium (air under normal conditions) of an environment in which diffraction grating 13 is used, and d is the grating constant of diffraction grating 13.
From equation (1), the relationship between the wavelength and the reflection angle is represented by equation (2) below.Δλ/Δθgo=na·d·cos θgo  (2)
As described above, even in the case of such light under measurement wherein a plurality of optical signal wavelengths are mixed, diffraction grating 13 reflects the components of the light at different, wavelength-by-wavelength angles, so that the components of the light under measurement are converged at the differently positioned light receiving elements of PDM 15 to form images. Consequently, it is possible to determine the wavelengths of individual optical signals.
It should be noted that in order to be able to obtain desired wavelengths, the refractive index of the medium (air) must be constant. However, if any of such factors of the environment of use as the altitude above sea level, atmospheric pressure, temperature, and steam pressure differs, the refractive index of air also changes. For this reason, the angle of reflection from diffraction grating 13 changes even if the wavelengths of the light under measurement remain the same.
From equation (1), a change in the angle of reflection for a change in the refractive index of a medium is represented by equation (3) below.Δθgo/Δna=−λ/(na2·d·cos θgo)  (3)
For example, assume that λ=1.55 [μm], d=1.111 [μm], na=1.000268, and θgo=1.248 [rad] (71.5 [deg]). Then, from equation (3), Δθgo/Δna≈−4.42 holds true.
Consequently, even if the refractive index na of air changes by only as small as 0.00001 (equivalent to a change in altitude above sea level from 0 [m] to approximately 300 [m]) from 1.000268 to 1.000258, the angle of reflection changes by as much as 0.0442 [mrad]. This amount of change is equivalent to a wavelength of 15.5 [pm], according to equation (2).
This means that even if the wavelengths remain the same, the positions of images on PDM 15 also change as the refractive index of air changes.
As a result wavelength calculation means 30 calculates the wavelengths of the light under measurement from the positions of images on PDM 15, the accuracy of wavelength measurement deteriorates.